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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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CHILDS  GLACIER                                         407
lay upon the serac tops (PI. CLV). These were, at the time of our visit, continually sliding down into the cracks, for early in 1910, this margin was so little broken that some of the bridge engineers had no difficulty in walking over it; but by August it was so crevassed that we found it impossible to ascend the precipit&us glacier margin or to walk more than a few yards upon the ice. We, therefore, assume that nearly all of the crevassing took place during the spring and early summer of 1910.
Despite the great forward movement, however, the lateral spreading did not result in severe crevassing in all of the northern margin of the glacier, for the extreme northern margin was still thickly covered with ablation moraine (PI. CLJJLJL) with crevasses extending into the edge of it and not affecting the outer portion, where a few small shrubs still grew upon the ice in August.
There were notable variations of drainage along the northern margin. In 1909, and for some time before, there was a small marginal stream in front of the glacier, which flowed southward to the Copper River at high water, entering the river directly; but at low wafer stages, the main portion of this marginal stream flowed eastward, behind a gravel bar, nearly to the railway bridge before entering the westward-flowing Copper River. On June 10,1910, this stream was at the very edge of the glacier, but a little earlier some of the engineers observed that the glacier was 75 to 100 feet west of the stream. The stream did not maintain a marginal position in front of the advancing glacier, however, for on July 29 there was no stream visible. At the time of our visits, in August, a marginal lake had formed, covering part of the northern alluvial fan and extending eastward nearly half a mile into the forest, where the water stood waist-deep among the trees. This lake ended 200 or 800 yards north of Copper River and had an invisible subglacial outlet. Between September 19 and October 5 the outlet, then visible, was extended over 200 feet southward, cutting back the glacier margin some distance, and then continuing in a subglacial course to the Copper River.
The rate of movement in mid-glacier during the summer was not determined, but judging by the measured rate at the northern margin, and by comparison with measured rates at intervals across glaciers in other regions, it may well have been at least six times the 1909 rate, that is SO or 40 feet a day, or at times even more. But the middle of the glacier did not advance as far during the summer as the northern margin did, and during 16 days in May, when the river was rising, it retreated 200 feet, while between June 3 and August 11 there was a retreat of 415 feet, and between August 11 and 17 of 160 feet. This retreat occurred while the northern margin was advancing most strongly and it seemed to us that it was not due to a cessation of forward motion but wholly to undercutting by the Copper River, which rose six feet between June and August. We accordingly predicted in August that the ice front in the river would readvance during the latter part of September when the river was lower. This prediction proved correct, for up to October 5 there was an advance of 825 feet in mid-glacier plus whatever additional retreat occurred between August 17 and the date when the advance commenced, the river having fallen over nine feet in the meantime. The rate of retreat from August 11 to 17 was at least 27 feet a day.
Since the glacier advanced at least 825 feet in the 49 days between August 17 and October 5 the rate of actual advance in mid-glacier was over 6^ feet a day. As (a) icebergs were discharging all the time, as (b) the advance was surely over 325 feet, and as (c) the forward movement began not on August 17 but probably after the